The invention is based on a hydraulic dual-circuit brake system with an anti-skid system (ABS) and traction control (ASR) for motor vehicles, as defined hereinafter.
In a known dual-circuit brake system of this type German Patent 38 16 073 now U.S. Pat. No. 4,900,102 having front-axle/rear-axle or front/rear brake circuit distribution, the output of the precharging pump communicates with the inlet of whichever pump element of the return pump is operative in the brake circuit of the driven wheels. The valve unit has a 3/2-way magnet valve, with three controlled valve connections, of which the first valve connection communicates with the outlet of the aforementioned pump element; the second valve connection communicates with the brake circuit outlet of the master brake cylinder associated with the brake circuit of the driven wheel; and the third valve connection communicates with the brake fluid tank, via the pressure limiting valve. The 3/2-way magnet valve is embodied such that in its basic position, the first valve connection communicates with the second valve connection, and in its reversed position brought about in traction control, it communicates with the third valve connection. In traction control, the turned-on precharging pump feeds brake fluid into the pump element of the return pump associated with the brake circuit of the driven wheels; in turn, this pump element generates the high brake pressure to apply the brake calipers and slow down the spinning driven wheel. The brake pressure required for this braking, i.e. slowing down of this wheel, is established by brake pressure modulation, as a result of switching of the control valve associated with the spinning driven wheel. Excess brake fluid is pumped back to the brake fluid tank via the pressure limiting valve. In the charging process of the pump element, the low-pressure reservoir connected to the inlet of the pump element is likewise filled, so that in the process of filling the wheel brake cylinder associated with the spinning driven wheel, a considerable volume of brake fluid is withdrawn This prolongs the filling time for the wheel brake cylinder, so that the brake calipers cannot slow down the spinning driven wheel until after an idle time, which especially with large-volume wheel brake cylinders is unacceptably long. The dynamics of the dual circuit brake system are accordingly unsatisfactory.